The present invention relates to lasers and more particularly to a disk laser including an amplified spontaneous emission suppression (ASE) feature and a method for making the disk laser.
Amplified spontaneous emission (ASE) is a phenomenon wherein spontaneously emitted photons traverse a laser gain medium or laser host material and may be amplified (multiplied) before they exit the gain medium volume. A favorable condition for ASE is a combination of high gain and a long path for the spontaneously emitted photons. ASE may depopulate the upper energy level in an excited laser gain medium and may rob the laser of its power. Additionally, reflection of ASE photons at gain medium boundaries may provide feedback for parasitic oscillations that aggravate the loss of laser power. If unchecked, ASE may become large enough to deplete the upper level inversion in high-gain laser amplifiers. Furthermore, in certain disk lasers, such as ytterbium disk lasers and similar lasers, excessive ASE may lead to failure of the laser disk. Thus ineffective ASE suppression may require operating the laser disk at a substantially lower than design gain and may reduce the robustness and reliability of the laser system.
FIG. 1 is an illustration of a prior art thin disk laser 100. The disk laser 100 may include a laser host material of yttrium aluminum garnet (YAG) doped with laser ions, such as trivalent ytterbium (Yb3+) ions which are known to have a laser transition in the vicinity of 1029 nm. A back face 106 of the disk laser 100 may be bonded to a heat sink 108.
A front face 104 of the Yb:YAG disk laser 100 may receive pump radiation 102 at about 941 nm which is absorbed by the Yb3+ ions and excites them to a laser transition centered at about 1029 nm. The pump radiation 102 may be made to illuminate only a central portion 110 of the disk laser 100, as illustrated by the broken or dash lines in FIG. 1. Yb:YAG being a quasi-3 level material normally exhibits absorption of light in the vicinity of its peak lasing wavelength of 1029 nm. To overcome such absorption, pump radiation 102 may be sufficiently intense to make the Yb:YAG material in the disk laser 100 transparent (non-absorbing) at 1029 nm. Hence, the central portion 110 of the disk 100 may exhibit a net laser gain which makes it suitable for amplification of laser radiation in the vicinity of 1029 nm. On the other hand, an annular edge portion 112 of the disk 100 does not receive any substantial pump radiation 102. The disk 100 is monolithic and the central portion 110 and the edge portion 112 have the same doping and laser host material. As a result, the edge portion 112 not receiving any substantial pump radiation 102 may absorb radiation at 1029 nm. ASE radiation is emitted as the Yb3+ laser ions in the central portion 110 spontaneously decay from their excited state. Some portion of the ASE radiation may be trapped between the front surface 104 and the back surface 106 of the disk 100 and may travel in a zigzag-like path from the central portion 110 to the edge portion 112. If the amount of ASE radiation is rather small, the ASE radiation is effectively absorbed in the edge portion 112. In this fashion, the possibility for an ASE photon being reflected from a disk edge 114 (e.g., by Fresnel reflection) and being re-amplified in the central portion 110 is very remote. However, with increasing ASE intensity, such as may be experienced because of increased pumping and/or a non-lasing condition in the central portion 110, ASE photons entering the edge portion 112 may deplete the absorption property or capability of the edge portion 112 which is integral and homogeneous with the central portion 110 and has the same doping. Hence, the likelihood of ASE photons returning to and being re-amplified in the central portion 110 may be significantly increased. As a result, laser gain may be substantially depleted. If ASE intensity is further increased (e.g., due to increased pumping or due to a pause in lasing) parasitic lasing across the disk diameter may occur and the concomitant increase in thermal load may cause permanent damage to the laser disk 100.